1. Field of the Invention
The present invention relates generally to the assembly of micro-electronic devices and more particularly to thermocompression bonding of electrically conductive leads.
2. Description of the Prior Art
Various types of electronic components and even entire electronic circuits incorporating thousands of components are routinely manufactured in the form of an electronic component chip. Such chips generally have a central region containing electronic components which are, in many cases, electrically interconnected to form an entire electronic circuit. Electrical currents generally are introduced into and/or drawn from such components and/or circuits by means of electrical contact pads formed on the chip. These pads, which are electrically connected to the components and/or circuit on the chip, are generally square-shaped measuring between three and seven thousandths of an inch along an edge and are generally located along edges of the central region. However, due to the small size of the electrical contact pads and also because the electronic component chip may be easily damaged, larger and more rugged electrically conductive leads are generally permanently attached to these pads before the chip is placed in service.
U.S. Pat. Nos. 3,698,073, 3,698,074, 3,698,075 and 3,698,076 each relate to a method and apparatus whereby electrically conductive leads may be attached to the electrical contact pads of an electronic component chip. The technique taught by these patents employs a first, preformed lead frame fabricated from a strip of aluminum or copper having a thickness of about 1.5 to 4.0 thousandths of an inch. Electrically conductive leads preformed at the center of this strip are ultrasonically bonded to the electrical contact pads of an electronic component chip. Then these lead frames to which the chip is now bonded are attached to adhesive tape so that the outer, electrically shorting portions of the lead frame may be removed without damaging the leads bonded to the chip. The chips together with the leads from the frame remain secured to the tape which may then be wound onto a reel for sale or transport to another work position. Alternatively, the leads or a second, more rigid frame may be bonded to the leads of the first frame after which the chip and leads of the first frame may be removed from the adhesive tape. This second frame, to which the electronic component chip is now secured by means of the intermediate electrically conductive leads of the first frame, may be constructed of Kovar, nickel, copper, steel, or other suitable material having a thickness of six to twelve thousandths of an inch. This second lead frame is employed to provide leads having sufficient mechanical strength for ordinary handling and joining such as to printed circuit boards. A similar, commercially marketed technique for electronic component chip bonding employs a first lead frame fabricated in either a single-layer copper or a two-layer polyimide/copper preformed tape.
A difficulty with these prior techniques is that their apparent simplicity has not been realized in a manufacturing environment. For example, the fragile electrically conductive leads of the first preformed metallic tape must generally be protected by means of a backing web of insulating material. Similarly, the tools generally required to form the bonds between the electrically conductive leads and the electrical contact pads of the electronic component chip are complicated. For example, U.S. Pat. No. 3,698,075 teaches the use of ultrasonic bonding for this purpose wherein the pedestal against which the tape is pressed during bonding must be specially formed to have a relief portion in those areas where contacts are not to be formed and that its surface must be vapor honed in order to function properly. Also, the prior art techniques have often required either additional chip processing steps and/or specially shaped structures and complicated metallurgy at the terminal ends of the electrically conducted leads. Thus, it has been found necessary either to add an extra wafer-processing step to form a structure which prevents the pad from collapsing in normal thermocompression or alloy-type bonding or alternatively, the ends of the leads must be etched to form a bump thereon to mate with and contact the pad. Similarly, it has generally been found necessary to incorporate a layer of soft precious metal such as gold into the junction between the electrical contact pad and the electrically conductive leads in order to secure a bond which resists damage from temperature changes.
Another problem in bonding electrically conductive leads to electronic component chips left unsolved by prior art techniques is that of substrate bonding. Bonding an electrically conductive lead to the substrate into which and/or onto which the components and/or circuit are formed is frequently required for proper electrical operation of chips such as semiconductor integrated circuit chips. Since the substrate is most easily accessed only on the surface of the chip opposite to that on which the electrical contact pads are located, attachment of a lead thereto has not been previously accomplished with a method and apparatus for bonding preformed metallic tapes.